c++: Drop alignas restriction for stack variables.

Since expand_stack_vars and such know how to deal with variables aligned
beyond MAX_SUPPORTED_STACK_ALIGNMENT, we shouldn't reject alignas of large
alignments.  And if we don't do that, there's no point in having
check_cxx_fundamental_alignment_constraints at all, since
check_user_alignment already enforces MAX_OFILE_ALIGNMENT.

	PR c++/89357
	* c-attribs.c (check_cxx_fundamental_alignment_constraints): Remove.

gcc/c-family/ChangeLog

+2020-01-29  Jason Merrill  <jason@redhat.com>
+
+	PR c++/89357
+	* c-attribs.c (check_cxx_fundamental_alignment_constraints): Remove.
+
 2020-01-23  Jason Merrill  <jason@redhat.com>
 
 	* c-warn.c (conversion_warning): Change -Wsign-conversion handling.

gcc/c-family/c-attribs.c

@@ -1957,65 +1957,6 @@ fail:
   return res;
 }
 
-/* If in c++-11, check if the c++-11 alignment constraint with respect
-   to fundamental alignment (in [dcl.align]) are satisfied.  If not in
-   c++-11 mode, does nothing.
-
-   [dcl.align]2/ says:
-
-   [* if the constant expression evaluates to a fundamental alignment,
-   the alignment requirement of the declared entity shall be the
-   specified fundamental alignment.
-
-   * if the constant expression evaluates to an extended alignment
-   and the implementation supports that alignment in the context
-   of the declaration, the alignment of the declared entity shall
-   be that alignment
-
-   * if the constant expression evaluates to an extended alignment
-   and the implementation does not support that alignment in the
-   context of the declaration, the program is ill-formed].  */
-
-static bool
-check_cxx_fundamental_alignment_constraints (tree node,
-					     unsigned align_log,
-					     int flags)
-{
-  bool alignment_too_large_p = false;
-  unsigned requested_alignment = (1U << align_log) * BITS_PER_UNIT;
-  unsigned max_align = 0;
-
-  if ((!(flags & ATTR_FLAG_CXX11) && !warn_cxx_compat)
-      || (node == NULL_TREE || node == error_mark_node))
-    return true;
-
-  if (cxx_fundamental_alignment_p (requested_alignment))
-    return true;
-
-  if (VAR_P (node))
-    {
-      if (TREE_STATIC (node) || DECL_EXTERNAL (node))
-	/* For file scope variables and static members, the target supports
-	   alignments that are at most MAX_OFILE_ALIGNMENT.  */
-	max_align = MAX_OFILE_ALIGNMENT;
-      else
-	/* For stack variables, the target supports at most
-	   MAX_STACK_ALIGNMENT.  */
-	max_align = MAX_STACK_ALIGNMENT;
-      if (requested_alignment > max_align)
-	alignment_too_large_p = true;
-    }
-  /* Let's be liberal for types and fields; don't limit their alignment any
-     more than check_user_alignment already did.  */
-
-  if (alignment_too_large_p)
-    pedwarn (input_location, OPT_Wattributes,
-	     "requested alignment %d is larger than %d",
-	     requested_alignment / BITS_PER_UNIT, max_align / BITS_PER_UNIT);
-
-  return !alignment_too_large_p;
-}
-
 /* Common codes shared by handle_warn_if_not_aligned_attribute and
    handle_aligned_attribute.  */
 
@@ -2059,8 +2000,7 @@ common_handle_aligned_attribute (tree *node, tree name, tree args, int flags,
   /* Log2 of specified alignment.  */
   int pow2align = check_user_alignment (align_expr, objfile,
 					/* warn_zero = */ true);
-  if (pow2align == -1
-      || !check_cxx_fundamental_alignment_constraints (*node, pow2align, flags))
+  if (pow2align == -1)
     {
       *no_add_attrs = true;
       return NULL_TREE;

gcc/testsuite/g++.dg/cpp0x/alignas17.C

+// PR c++/89357
+// { dg-do compile { target c++11 } }
+
+void g(int &);
+
+void f0() {
+  __attribute__((aligned(128))) static int x;
+  g(x);
+}
+
+void f1() {
+  alignas(128) int x;
+  g(x);
+}